Genetic probes capable of recognizing differences in the VP4 and VP7 genes of rotaviruses belonging to different serotypes have been derived from a variety of these genes and employed to identify the serotype specificity of rotavirus field strains. These probes were synthesized from either rotavirus RNA or cloned DNA copies of the fourth (VP4) or ninth gene (VP7) by amplification of specific regions (300-400 bases long) which were divergent among serotypes using the "polymerase chain reaction" (PCR). The correct serotype (VP4 and VP7) of several well characterized laboratory strains was confirmed using these probes. The VP7 specificity of several rotavirus field strains from different sources was also identified with these probes. The results were similar to those obtained with monoclonal antibody ELISA. The VP4 specificities of the same field isolates have also been determined. This latter result is of importance since monoclonal antibodies are currently not available to classify the VP4 antigenic specificity of different rotavirus strains. "Serotyping" rotaviruses by this approach may be of practical benefit in epidemiological studies and should increase our understanding the natural history of rotaviruses. It will also aid in the evaluation of rotavirus vaccine candidates.